The present invention relates to a method for preparing sodium tripolyphosphate from a wet process phosphoric acid, and more particularly, the preparation of white sodium tripolyphosphate from the neutralization of wet process phosphoric acid with soda ash or sodium hydroxide.
Sodium tripolyphosphate (STPP), also known as pentasodium tripolyphosphate (Na.sub.5 P.sub.3 O.sub.10), is one of the major detergent builders in use today. The use of STPP as a detergent builder is to increase the cleaning ability of detergent compositions. It is made in larger quantities than any other high purity phosphate. The total U.S. production in 1974 exceeded 800,000 tons.
Compared to its use as a builder for detergents, other uses of Na.sub.5 P.sub.3 O.sub.10 become minor. However, large tonnages are consumed annually as deflocculants for solid slurries to reduce the amount of water needed as in the cases of cement manufacturing, oil well drilling mud formulations, and kaolin clay processing.
Anhydrous sodium tripolyphosphate occurs in two crystalline forms. Form II, the low temperature variety, is obtained at about 400.degree. C. It is usually converted to Form I, the thermodynamically stable phase and high temperature variety, by heating at a temperature of about 500.degree. to about 550.degree. C. The phase transition temperature of Form II to Form I is 417.degree. C. Reverse transition of Form I to Form II is difficult. See A. D. F. Toy's treatise on "Inorganic Phosphorus Chemistry" in Comprehensive Inorganic Chemistry, Vol. 2 Chapter, 20, page 515 (Pergamon Press 1973), edited by Bailar et al.
The hydrate of sodium tripolyphosphate, Na.sub.5 P.sub.3 O.sub.10.6H.sub.2 O, is formed by the addition of either Form I or Form II to water. Both anhydrous forms are metastable in water and cannot be recrystallized from water.
The major difference between the structures of Form I and Form II is in the ionic coordination of the sodium ions. In Form II all sodium ions are coordinated octahedrally by oxygen, while in Form I some sodium ions are surrounded by only four oxygen atoms. See Andon et al, J. Appl. Chem. (1967), Vol. 17, pages 65-70.
For detergent applications, the commercial sodium tripolyphosphate product is usually a mixture of Form I and Form II in a ratio determined by the needs of the specific detergent manufacture. In addition, commercial products also contain a few percent of sodium pyrophosphate (Na.sub.4 P.sub.2 O.sub.7). The latter is the result of using a slight excess of Na.sub.2 O over the theoretical Na.sub.2 O/P.sub.2 O.sub.5 ratio of 5:3 during the manufacturing process for the purpose of avoiding the formation of any insoluble sodium metaphosphate. The presence of even a minor amount of insoluble sodium metaphosphate in the product can cause turbidity in a water solution, a highly undesirable feature for detergent applications.
The major function of sodium tripolyphosphate (Na.sub.5 P.sub.3 O.sub.10) as a detergent builder depends upon its water-softening action through complexing or sequestering of calcium and magnesium ions in hard water. Another important function of sodium tripolyphosphate in detergents is its ability to suspend and peptize dirt particles. The tripolyphosphate anion is also important for its ability to lower the critical micelle concentration of the detergent.
The market today for detergent builders is quite diverse. The best known field of application for builders is in heavy duty, spray dried detergent formulations for household use. These widely advertised products generally contain about 25 to about 35% by weight of phosphates in the form of sodium tripolyphosphate or a mixture of sodium tripolyphosphate and tetrasodium pyrophosphate. In the household market, there are also low sudsing detergent formulations containing a sodium tripolyphosphate builder. Additionally, many light duty synthetic detergents, as well as the liquid detergents, contain sodium tripolyphosphate.
Through hydration in the synthetic detergent manufacturing process, except for a few percent of degradation products, all of the original Na.sub.5 P.sub.3 O.sub.10 is converted to Na.sub.5 P.sub.3 O.sub.10.6H.sub.2 O.
The general method for producing STPP is to react phosphoric acid (herein used to mean either the anhydride P.sub.2 O.sub.5 or the acid itself) and sodium carbonate (soda ash) together in an aqueous solution until the mole ratio of sodium to phosphorus is about 5:3. The reaction results in formation of an aqueous mixture containing monosodium orthophosphate (NaH.sub.2 PO.sub.4) and disodium orthophosphate (Na.sub.2 HPO.sub.4) in a mole ratio of about 1:2. The aqueous mixture is then dried to a powder of crystalline form. STPP is then formed by heating at a controlled temperature, in the range of about 250.degree. to about 600.degree. C. an intimate mixture of the disodium orthophosphate and monosodium orthophosphate in a 2 to 1 ratio in accordance with the following equation: EQU 2Na.sub.2 HPO.sub.4 +NaH.sub.2 PO.sub.4 .fwdarw.Nahd 5P.sub.3 O.sub.10 +2H.sub.2 O
The problems involved in making commercial sodium tripolyphosphate exhibiting (1) a high assay of Na.sub.5 P.sub.3 O.sub.10, (2) the desired ratio of crystalline Form 1 and Form 2, and (3) suitable physical properties, have led to a wide variety of processes. These include spray-drying the orthophosphate precursor prior to calcination; spray-drying the orthophosphate solution directly to the final product, as disclosed in Canadian Pat. No. 543,968; introducing the orthophosphate liquor directly into the calcine, as disclosed in U.S. Pat. No. 2,419,148; carrying out the continuous comminution during calcining as disclosed in U.S. Pat. No. 2,747,964; and even melting, chilling to give partial vitrification, and subsequently tempering as disclosed in U.S. Pat. No. 2,174,614.
Previously, phosphoric acid made by the "thermal method" was the only acid suitable for the reaction with soda ash to form the sodium orthophosphate mixture. Thermal acid is formed by the oxidation of elemental phosphorus to form phosphoric anhydride which is then hydrated to phosphoric acid. The thermal phosphoric acid, after being treated with hydrogen sulfide or sodium hydrosulfide to remove heavy metal impurities, is then diluted to 75, 80 or 85% phosphoric acid for commercial applications. Phosphoric acid made from thermal acid has a minimum of impurities, however, its use entails the requirement of relatively expensive capital equipment, large amounts of electrical energy, and pollution control. Because of the wide gap between the price of wet process phosphoric acid and the price of electro-thermal acid made from elemental phosphorus, increasing attention has been drawn to the use of wet process phosphoric acid as a substitute in those areas of technology where phosphoric acid made from the thermal method was principally utilized.
Among the disadvantages of using wet process phosphoric acid for the production of sodium tripolyphosphate is the fact that wet process phosphoric acid contains numerous impurities which impart undesirable color to sdium tripolyphosphate. For example, wet process phosphoric acid contains impurities such as organic matter, vanadium, and iron which can lead to a brown, yellow, green, gray, pink or other colored sodium tripolyphosphate product, depending upon the quantity and oxidation state of impurities present in the acid.
The present invention has achieved an efficient and commercially feasible method for producing sodium tripolyphosphate from wet process phosphoric acid. Moreover, the process of the present invention is characterized by a simple, efficient approach for producing white sodium tripolyphosphate from the neutralization of wet process phosphoric acid with soda ash or sodium hydroxide.